correct color scaling in vertex shader

Nitpicking, but fix for #1815 didn't get the ratio exactly right.

    float truncated = 0xFE / 255.0f;
    System.out.println(truncated * (256.0f / 255.0f)); // 0.9999847
    System.out.println(truncated * (255.0f / 254.0f)); // 1.0